A Tectonic Interpretation of Petrochemical Signatures of Proterozoic and Paleozoic Alkaline Rocks from the Porjaguba Dike Swarm, Kandalaksha Bay, White Sea

摘要

The Porjaguba dike swarm in the terminal part of the Onega-Kandalaksha Precambrian segment of the White Sea (Belomorian) rift system is the only cluster of lamproite dikes dated as Early Proterozoic (1720 +- 8 Ma). An examination of petrochemical signatures of the lamproites has shown their affinity to differentiates of a common alkaline ultrabasic mantle-derived magma. The systematic Bowen-type line of descent was established both in individual dikes from their margins toward their central zones and southeastward (seaward) along the strike of the on-land part of the dike swarm as a whole. On the discriminant diagrams, the lamproite data points fall into fields of within-plate basalts or basalts (including their derivatives) typical of destructive continental margins. This is also true of alkali basalts from particular Paleozoic dikes, although the latter are characterized by quite opposite trends of the within-dike fractionation. The data obtained are consistent with the results of structural studies, indicating predominant localization of the lamproite dikes in fractured zones, as well as with some specific morphological features of the dikes that suggest the dike emplacement in an exten-sional setting. This setting favored magma fractionation within dikes and, probably, in feeding chambers. Thus, the most plausible interpretation assumes that the Proterozoic dikes were formed in the axial part of the Precambrian paleorift system. The Paleozoic dikes were emplaced into a rift- structure characterized by a substantial strike-slip displacement. Alternation of transtension and transpression segments could have provided irregular spatial distribution of dikes and variable tectonic settings of their emplacement. A normal fractionation trend is observed in dikes from the transtension segments, whereas a quite different and rather specific trend is typical of the dikes localized in transpression segments, as is the case of the Porjaguba swarm. This difference can be explained by assimilation of the upper crustal felsic material by dikes emplaced under transpressive regime.